Feeding injury of Tetranychus urticae Koch (Acari, Tetranychidae) on bean plant Ieaves was examined using Iight and electron microscopy. Histological effects are directly related to the feeding intensity of the mites and are a function of time. The thickness of infested Ieaves is strongly reduced as a result of a reduction of both mesophyll layers. Cells which have been punctured by feeding mites collapse and exhibit no contents. Adjacent unpunctured cells show coagulated protoplasts and, as further damage, degenerative processes in the chloroplast structures. Thus, the feeding injury of Tetranychus is not limited to punctured ceils, but ceils of ail mesophyll layers are affected. Possible mechanisms are discussed.